Organosilicon polymers, as a class of engineering thermoplastics, have good heat resistance and excellent mechanical properties in the form of polyesters and polyamides. A precursor for polyesters and polyamides having organosilicon segments is necessarily a polyfunctional molecule containing an organosilicon moiety, preferably of a polycarboxylic character which can be reacted readily with an alcohol or an amine, etc., to produce a polyester or polyamide, etc. of suitable chain length. Such precursors include silicon-containing aromatic polyacids such as bis(p-carboxylphenyl)dimethylsilane.
Previous investigators have prepared bis(carboxyphenyl) silanes by oxidation of the corresponding bis(alkyaryl)tetravalent silicon compounds. Examples in the literature claim successful conversions with a variety of oxidation reagents and conditions.
Tyler, U.S. Pat. No. 2,517,146 teaches the oxidation of ditolydimethylsilane by heating the silane with alkaline potassium permanganate or by passing the silane at elevated temperatures in air over a metallic oxide catalyst such as manganese dioxide or chromium trioxide. A second method taught by Tyler is by carbonating the Grignard reagent of dibromophenyldimethylsilane. The Grignard reagent of the silane is reacted. with an excess of solid carbon dioxide powder. The resulting dicarboxyphenyldimethylsilane was reacted with polyhydric alcohols and polyfunctional amines to produce resins of the polyamide and alkyd types.
Speck, U.S. Pat. No. 2,722,524, teaches hydrolysis of bis(p-cyanophenyl) dimethylsilane with potassium hydroxide in ethyl alcohol and water to prepare the diacid. Speck also teaches the oxidation of ditolydiphenylsilane with potassium permanganate in a hot aqueous pyridine system. The hexamethylenediamine salt of the bis(p-carboxyphenyl) dimethylsilane was polymerized by the salt fusion technique. The resulting polymer was a tough, clear colorless film. Polyester fibers prepared from the diacid with ethylene glycol had good initial tensile modulus, elastic recovery, orientation along the fiber axis, as well as good strength and dyeing characteristics.
Japanese Patent Publication No. 310846/1988 teaches a batch process for production of a diaryl dicarboxylic acid containing a substituted carbon or substituted silicon atom by oxidizing a corresponding dialkyl-substituted diaryl compound with molecular oxygen in the presence of a catalyst comprising cobalt, bromine and chlorine, and a heavy metal preferably selected from the group consisting of manganese, cerium, zirconium, chromium and nickel. A 95% yield was reported of bis(4-carboxyphenyl)dimethylsilane in the presence of a cobalt, manganese, bromine, and chlorine catalyst wherein the mole ratios of the catalyst components were 48:4:46:48 millimoles, respectively.
Despite the above-reported methods of preparing bis(4-carboxylphenyl) dimethylsilane, each process has disadvantages in the manufacture of the diacid at a low cost on an industrial scale. The permanganate method results in the production of undesirable by-products, including manganese dioxide. The reaction of the Grignard reagent with solid carbon dioxide requires the use of the Grignard reaction to prepare the precursor. It also results in a large amount of undesirable products. The oxidation of a bis(alkylphenyl) dialkylsilane in the presence of a catalyst comprising chlorine in the form of concentrated hydrochloric acid requires special corrosion-resistant process equipment.
Accordingly, it is an object of this invention to provide an improved method which overcomes the aforesaid problems of prior art methods by the liquid phase oxidation of an alkyl-substituted diaryl dialkyl silane with an oxygen-containing gas in a solvent and in the presence of an oxidation catalyst.
More particularly, it is an object of this invention to provide an improved method for preparing bis(p-carboxyphenyl) dimethylsilane and bis(3,4-dicarboxyphenyl)dimethylsilane in the presence of a cobalt-manganese-bromine catalyst wherein problems of corrosion of process equipment are overcome in a low-cost, economical, industrially advantageous process which is suitable for large scale production of silicon-containing aromatic polyacids.